Ever since Sarah Simmons started City Grit in September 2011, guest chefs, pop-ups and chef collaborations have gained in popularity.

Alex Stupak of Empellon did a series this past November with chefs Wylie Dufresne (Alder and WD-50), Ken Oringer and Andrew Brochu, among others, David Santos at Louro hosts various chefs nearly every Monday and Daniel Humm of Eleven Madison Park switched restaurants with Chicago chef Grant Achatz of Alinea last summer. Now, with the forthcoming summer opening of RES, an artist residency for chefs by Frank Falcinelli and Frank Castronovo of Frankies Spuntino and Prime Meats, and the pop-up space Fatty Lab by the team at Fatty Crew Hospitality Group, even more chefs are showing interest in this type of partnership.

"Pop-ups are a great creative outlet to express ideas without having to commit to opening a restaurant, and we want to provide that outlet," said Fatty Crew partner Colin Camac, who hosts guests in the old Fatty 'Cue spot in Williamsburg. "We choose our pop-up chefs from people we have worked with in the past or would like to work with in the future."

In December, Chaz Brown, former chef de cuisine of Fatty Crab and latest star of Bravo's Top Chef, kicked off Fatty Lab and served up an array of Brooklyn-meets-Creole-style dishes, including spicy head-on shrimp, gumbo and collards laced with pork. Though they won't divulge their next guest, the group plans to keep running Fatty Lab for a while, and between stints, they will be busy recipe testing and holding private events.

Another way chefs have been teaming up is through special projects, like the Guest Chefs Give Back program at the popular sandwich shops Num Pang. Founders Ratha Chaupoly and Ben Daitz started this series in February 2012 with Mario Batali, who created the Batali Pang sandwich. This sausage, pickled balsamic onion and Cacio de Roma cheese creation not only showcased Batali's skill at making a sandwich within the confines of Num Pang's tiny kitchen, but also benefited their respected charities. The last collaboration they offered in December was the Gramercy Tavern Pang, featuring Gramercy Tavern chef Michael Anthony and a sandwich inspired by the restaurant's new cookbook.

"In our opinion, these guys are the best chefs in the city hands down, and also great guys willing to give their time and creativity for all these great causes," said Daitz. "Also, it's fun to work with people that are doing something different than what we are doing."

This exact concept is why Simmons started City Grit in the first place.

"I would meet a chef and the first thing they would say, 'Oh my god, I would love to come cook in New York,' so I started to realize there was a demand to have a place where they could cook," she said. "The wheels started turning, and it started out really as something selfish, because I wanted a way to not cook every single day."

Hence, City Grit was born inside a furniture store that is within an old school, and has hosted talent from Sean Brock to April Bloomfield to Paul Qui. Yet despite the excitement and team effort, hosting a guest chef isn't always easy.

"I think a lot of people don't work with guest chefs on a regular basis because it's a lot of work," said Simmons. "We've designed a system for executing these events and it's still hard, like opening a new restaurant every 48 hours."

Of course, this also means guests get to experience something unique each time, which is part of the pull, and one that's clearly strong enough to inspire other chef collaborations all over the city.